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<!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>性能优化</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>手写代码</span> <span class="arrow right"></span></p> <!----></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="面试官-说说你对函数式编程的理解-优缺点"><a href="#面试官-说说你对函数式编程的理解-优缺点" class="header-anchor">#</a> 面试官：说说你对函数式编程的理解？优缺点？</h1> <p><img src="https://static.vue-js.com/ec0f6e80-8534-11eb-85f6-6fac77c0c9b3.png" alt=""></p> <h3 id="_10-函数式编程"><a href="#_10-函数式编程" class="header-anchor">#</a> 10. 函数式编程</h3> <ul><li><p><a href="https://juejin.im/post/6844903936378273799#heading-32" target="_blank" rel="noopener noreferrer">简明 JavaScript 函数式编程——入门篇<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p></li> <li><p><a href=""></a></p></li> <li><p>柯里化（Curry）: 柯里化的意思是将一个多元函数，转换成一个依次调用的单元函数。</p></li> <li><p>函数组合（Compose）: 函数组合的目的是将多个函数组合成一个函数</p></li></ul> <p>函数式编程的优点：</p> <ul><li>代码简洁，开发快速：函数式编程大量使用函数的组合，函数的复用率很高，减少了代码的重复，因此程序比较短，开发速度较快。</li> <li>接近自然语言，易于理解：函数式编程大量使用声明式代码，基本都是接近自然语言的，加上它没有乱七八糟的循环，判断的嵌套，因此特别易于理解。</li> <li>易于&quot;并发编程&quot;：函数式编程没有副作用，所以函数式编程不需要考虑“死锁”（Deadlock），所以根本不存在“锁”线程的问题。</li> <li>更少的出错概率：因为每个函数都很小，而且相同输入永远可以得到相同的输出，因此测试很简单，同时函数式编程强调使用纯函数，没有副作用，因此也很少出现奇怪的 Bug。</li></ul> <h2 id="一、是什么"><a href="#一、是什么" class="header-anchor">#</a> 一、是什么</h2> <p>函数式编程是一种&quot;编程范式&quot;（programming paradigm），一种编写程序的方法论</p> <p>主要的编程范式有三种：命令式编程，声明式编程和函数式编程</p> <p>相比命令式编程，函数式编程更加强调程序执行的结果而非执行的过程，倡导利用若干简单的执行单元让计算结果不断渐进，逐层推导复杂的运算，而非设计一个复杂的执行过程</p> <p>举个例子，将数组每个元素进行平方操作，命令式编程与函数式编程如下</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token comment">// 命令式编程</span>
<span class="token keyword">var</span> array <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span>
<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> array<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    array<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">=</span> Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span>array<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>

<span class="token comment">// 函数式方式</span>
<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token parameter">num</span> <span class="token operator">=&gt;</span> Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span>num<span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
</code></pre></div><p>简单来讲，就是要把过程逻辑写成函数，定义好输入参数，只关心它的输出结果</p> <p>即是一种描述集合和集合之间的转换关系，输入通过函数都会返回有且只有一个输出值</p> <p><img src="https://static.vue-js.com/f9f83900-8534-11eb-85f6-6fac77c0c9b3.png" alt=""></p> <p>可以看到，函数实际上是一个关系，或者说是一种映射，而这种映射关系是可以组合的，一旦我们知道一个函数的输出类型可以匹配另一个函数的输入，那他们就可以进行组合</p> <h2 id="二、概念"><a href="#二、概念" class="header-anchor">#</a> 二、概念</h2> <h3 id="纯函数"><a href="#纯函数" class="header-anchor">#</a> 纯函数</h3> <p>函数式编程旨在尽可能的提高代码的无状态性和不变性。要做到这一点，就要学会使用无副作用的函数，也就是纯函数</p> <p>纯函数是对给定的输入返还相同输出的函数，并且要求你所有的数据都是不可变的，即纯函数=无状态+数据不可变</p> <p><img src="https://static.vue-js.com/04f50720-8535-11eb-ab90-d9ae814b240d.png" alt=""></p> <p>举一个简单的例子</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">let</span> <span class="token function-variable function">double</span> <span class="token operator">=</span> <span class="token parameter">value</span><span class="token operator">=&gt;</span>value<span class="token operator">*</span><span class="token number">2</span><span class="token punctuation">;</span>
</code></pre></div><p>特性：</p> <ul><li>函数内部传入指定的值，就会返回确定唯一的值</li> <li>不会造成超出作用域的变化，例如修改全局变量或引用传递的参数</li></ul> <p>优势：</p> <ul><li>使用纯函数，我们可以产生可测试的代码</li></ul> <div class="language-js extra-class"><pre class="language-js"><code><span class="token function">test</span><span class="token punctuation">(</span><span class="token string">'double(2) 等于 4'</span><span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=&gt;</span> <span class="token punctuation">{</span>
  <span class="token function">expect</span><span class="token punctuation">(</span><span class="token function">double</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">toBe</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre></div><ul><li><p>不依赖外部环境计算，不会产生副作用，提高函数的复用性</p></li> <li><p>可读性更强 ，函数不管是否是纯函数  都会有一个语义化的名称，更便于阅读</p></li> <li><p>可以组装成复杂任务的可能性。符合模块化概念及单一职责原则</p></li></ul> <h3 id="高阶函数"><a href="#高阶函数" class="header-anchor">#</a> 高阶函数</h3> <p>在我们的编程世界中，我们需要处理的其实也只有“数据”和“关系”，而关系就是函数</p> <p>编程工作也就是在找一种映射关系，一旦关系找到了，问题就解决了，剩下的事情，就是让数据流过这种关系，然后转换成另一个数据，如下图所示</p> <p><img src="https://static.vue-js.com/104af1c0-8535-11eb-ab90-d9ae814b240d.png" alt=""></p> <p>在这里，就是高阶函数的作用。高级函数，就是以函数作为输入或者输出的函数被称为高阶函数</p> <p>通过高阶函数抽象过程，注重结果，如下面例子</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> <span class="token function-variable function">forEach</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">arr<span class="token punctuation">,</span>fn</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">let</span> i<span class="token operator">=</span><span class="token number">0</span><span class="token punctuation">;</span>i<span class="token operator">&lt;</span>arr<span class="token punctuation">.</span>length<span class="token punctuation">;</span>i<span class="token operator">++</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token function">fn</span><span class="token punctuation">(</span>arr<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">let</span> arr <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span><span class="token number">2</span><span class="token punctuation">,</span><span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token function">forEach</span><span class="token punctuation">(</span>arr<span class="token punctuation">,</span><span class="token punctuation">(</span><span class="token parameter">item</span><span class="token punctuation">)</span><span class="token operator">=&gt;</span><span class="token punctuation">{</span>
    console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>item<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span>
</code></pre></div><p>上面通过高阶函数 <code>forEach</code>来抽象循环如何做的逻辑，直接关注做了什么</p> <p>高阶函数存在缓存的特性，主要是利用闭包作用</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> <span class="token function-variable function">once</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">fn</span><span class="token punctuation">)</span><span class="token operator">=&gt;</span><span class="token punctuation">{</span>
    <span class="token keyword">let</span> done <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token operator">!</span>done<span class="token punctuation">)</span><span class="token punctuation">{</span>
            <span class="token function">fn</span><span class="token punctuation">.</span><span class="token function">apply</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span>fn<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span><span class="token keyword">else</span><span class="token punctuation">{</span>
            console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">&quot;该函数已经执行&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        done <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><h3 id="柯里化"><a href="#柯里化" class="header-anchor">#</a> 柯里化</h3> <p>柯里化是把一个多参数函数转化成一个嵌套的一元函数的过程</p> <p>一个二元函数如下：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">let</span> <span class="token function-variable function">fn</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span>y</span><span class="token punctuation">)</span><span class="token operator">=&gt;</span>x<span class="token operator">+</span>y<span class="token punctuation">;</span>
</code></pre></div><p>转化成柯里化函数如下：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> <span class="token function-variable function">curry</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">fn</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">y</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
            <span class="token keyword">return</span> <span class="token function">fn</span><span class="token punctuation">(</span>x<span class="token punctuation">,</span>y<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">let</span> myfn <span class="token operator">=</span> <span class="token function">curry</span><span class="token punctuation">(</span>fn<span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span> <span class="token function">myfn</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>上面的<code>curry</code>函数只能处理二元情况，下面再来实现一个实现多参数的情况</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token comment">// 多参数柯里化；</span>
<span class="token keyword">const</span> <span class="token function-variable function">curry</span> <span class="token operator">=</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">fn</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">function</span> <span class="token function">curriedFn</span><span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>args</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">if</span><span class="token punctuation">(</span>args<span class="token punctuation">.</span>length<span class="token operator">&lt;</span>fn<span class="token punctuation">.</span>length<span class="token punctuation">)</span><span class="token punctuation">{</span>
            <span class="token keyword">return</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
                <span class="token keyword">return</span> <span class="token function">curriedFn</span><span class="token punctuation">(</span><span class="token operator">...</span>args<span class="token punctuation">.</span><span class="token function">concat</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token operator">...</span>arguments<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span>
        <span class="token keyword">return</span> <span class="token function">fn</span><span class="token punctuation">(</span><span class="token operator">...</span>args<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">const</span> <span class="token function-variable function">fn</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">x<span class="token punctuation">,</span>y<span class="token punctuation">,</span>z<span class="token punctuation">,</span>a</span><span class="token punctuation">)</span><span class="token operator">=&gt;</span>x<span class="token operator">+</span>y<span class="token operator">+</span>z<span class="token operator">+</span>a<span class="token punctuation">;</span>
<span class="token keyword">const</span> myfn <span class="token operator">=</span> <span class="token function">curry</span><span class="token punctuation">(</span>fn<span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token function">myfn</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>关于柯里化函数的意义如下：</p> <ul><li>让纯函数更纯，每次接受一个参数，松散解耦</li> <li>惰性执行</li></ul> <h3 id="组合与管道"><a href="#组合与管道" class="header-anchor">#</a> 组合与管道</h3> <p>组合函数，目的是将多个函数组合成一个函数</p> <p>举个简单的例子：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">function</span> <span class="token function">afn</span><span class="token punctuation">(</span><span class="token parameter">a</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">return</span> a<span class="token operator">*</span><span class="token number">2</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">function</span> <span class="token function">bfn</span><span class="token punctuation">(</span><span class="token parameter">b</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">return</span> b<span class="token operator">*</span><span class="token number">3</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">const</span> <span class="token function-variable function">compose</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter">a<span class="token punctuation">,</span>b</span><span class="token punctuation">)</span><span class="token operator">=&gt;</span><span class="token parameter">c</span><span class="token operator">=&gt;</span><span class="token function">a</span><span class="token punctuation">(</span><span class="token function">b</span><span class="token punctuation">(</span>c<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> myfn <span class="token operator">=</span>  <span class="token function">compose</span><span class="token punctuation">(</span>afn<span class="token punctuation">,</span>bfn<span class="token punctuation">)</span><span class="token punctuation">;</span>
console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span> <span class="token function">myfn</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>可以看到<code>compose</code>实现一个简单的功能：形成了一个新的函数，而这个函数就是一条从 <code>bfn -&gt; afn</code> 的流水线</p> <p>下面再来看看如何实现一个多函数组合：</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> <span class="token function-variable function">compose</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>fns</span><span class="token punctuation">)</span><span class="token operator">=&gt;</span><span class="token parameter">val</span><span class="token operator">=&gt;</span>fns<span class="token punctuation">.</span><span class="token function">reverse</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">acc<span class="token punctuation">,</span>fn</span><span class="token punctuation">)</span><span class="token operator">=&gt;</span><span class="token function">fn</span><span class="token punctuation">(</span>acc<span class="token punctuation">)</span><span class="token punctuation">,</span>val<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p><code>compose</code>执行是从右到左的。而管道函数，执行顺序是从左到右执行的</p> <div class="language-js extra-class"><pre class="language-js"><code><span class="token keyword">const</span> <span class="token function-variable function">pipe</span> <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token parameter"><span class="token operator">...</span>fns</span><span class="token punctuation">)</span><span class="token operator">=&gt;</span><span class="token parameter">val</span><span class="token operator">=&gt;</span>fns<span class="token punctuation">.</span><span class="token function">reduce</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token parameter">acc<span class="token punctuation">,</span>fn</span><span class="token punctuation">)</span><span class="token operator">=&gt;</span><span class="token function">fn</span><span class="token punctuation">(</span>acc<span class="token punctuation">)</span><span class="token punctuation">,</span>val<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p>组合函数与管道函数的意义在于：可以把很多小函数组合起来完成更复杂的逻辑</p> <h2 id="三、优缺点"><a href="#三、优缺点" class="header-anchor">#</a> 三、优缺点</h2> <h4 id="优点"><a href="#优点" class="header-anchor">#</a> 优点</h4> <ul><li><p>更好的管理状态：因为它的宗旨是无状态，或者说更少的状态，能最大化的减少这些未知、优化代码、减少出错情况</p></li> <li><p>更简单的复用：固定输入-&gt;固定输出，没有其他外部变量影响，并且无副作用。这样代码复用时，完全不需要考虑它的内部实现和外部影响</p></li> <li><p>更优雅的组合：往大的说，网页是由各个组件组成的。往小的说，一个函数也可能是由多个小函数组成的。更强的复用性，带来更强大的组合性</p></li> <li><p>隐性好处。减少代码量，提高维护性</p></li></ul> <h4 id="缺点"><a href="#缺点" class="header-anchor">#</a> 缺点：</h4> <ul><li><p>性能：函数式编程相对于指令式编程，性能绝对是一个短板，因为它往往会对一个方法进行过度包装，从而产生上下文切换的性能开销</p></li> <li><p>资源占用：在 JS 中为了实现对象状态的不可变，往往会创建新的对象，因此，它对垃圾回收所产生的压力远远超过其他编程方式</p></li> <li><p>递归陷阱：在函数式编程中，为了实现迭代，通常会采用递归操作</p></li></ul> <h2 id="参考文献"><a href="#参考文献" class="header-anchor">#</a> 参考文献</h2> <ul><li>https://zhuanlan.zhihu.com/p/81302150</li></ul></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">更新时间:</span> <span class="time">12/14/2021, 10:12:35 AM</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev">
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